A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
It has been proposed to immerse the substrate in the lithographic projection apparatus in a liquid having a relatively high refractive index, e.g. water, so as to fill a space between the final element of the projection system and the substrate. The point of this is to enable imaging of smaller features since the exposure radiation will have a shorter wavelength in the liquid. (The effect of the liquid may also be regarded as increasing the effective NA of the system and also increasing the depth of focus.) Other immersion liquids have been proposed, including water with solid particles (e.g. quartz) suspended therein.
However, submersing the substrate or substrate and substrate table in a bath of liquid (see, for example, U.S. Pat. No. 4,509,852, hereby incorporated in its entirety by reference) means that there is a large body of liquid that must be accelerated during a scanning exposure. This requires additional or more powerful motors and turbulence in the liquid may lead to undesirable and unpredictable effects.
One of the solutions proposed is for a liquid supply system to provide liquid on only a localized area of the substrate and in between the final element of the projection system and the substrate using a liquid confinement system (the substrate generally has a larger surface area than the final element of the projection system). One way which has been proposed to arrange for this is disclosed in PCT patent application publication WO 99/49504, hereby incorporated in its entirety by reference. As illustrated in FIGS. 2 and 3, liquid is supplied by at least one inlet IN onto the substrate, preferably along the direction of movement of the substrate relative to the final element, and is removed by at least one outlet OUT after having passed under the projection system. That is, as the substrate is scanned beneath the element in a −X direction, liquid is supplied at the +X side of the element and taken up at the −X side. FIG. 2 shows the arrangement schematically in which liquid is supplied via inlet IN and is taken up on the other side of the element by outlet OUT which is connected to a low pressure source. In the illustration of FIG. 2 the liquid is supplied along the direction of movement of the substrate relative to the final element, though this does not need to be the case. Various orientations and numbers of in- and out-lets positioned around the final element are possible, one example is illustrated in FIG. 3 in which four sets of an inlet with an outlet on either side are provided in a regular pattern around the final element.
A further immersion lithography solution with a localized liquid supply system is shown in FIG. 4. Liquid is supplied by two groove inlets IN on either side of the projection system PL and is removed by a plurality of discrete outlets OUT arranged radially outwardly of the inlets IN. The inlets IN and OUT can be arranged in a plate with a hole in its center and through which the projection beam is projected. Liquid is supplied by one groove inlet IN on one side of the projection system PL and removed by a plurality of discrete outlets OUT on the other side of the projection system PL, causing a flow of a thin film of liquid between the projection system PL and the substrate W. The choice of which combination of inlet IN and outlets OUT to use can depend on the direction of movement of the substrate W (the other combination of inlet IN and outlets OUT being inactive).
Another solution which has been proposed is to provide the liquid supply system with a barrier member which extends along at least a part of a boundary of the space between the final element of the projection system and the substrate table, as depicted in FIG. 5. The barrier member is substantially stationary relative to the projection system in the XY plane though there may be some relative movement in the Z direction (in the direction of the optical axis). A seal is formed between the barrier member and the surface of the substrate. In an embodiment, the seal is a contactless seal such as a gas seal. Such a system with a gas seal is disclosed in U.S. patent application publication no. US 2004-0207824, hereby incorporated in its entirety by reference.
In European patent application publication no. EP 1420300 and U.S. patent application publication no. US 2004-0136494, each hereby incorporated in their entirety by reference the idea of a twin or dual stage immersion lithography apparatus is disclosed. Such an apparatus is provided with two tables for supporting the substrate. Leveling measurements are carried out with a table at a first position, without immersion liquid, and exposure is carried out with a table at a second position, where immersion liquid is present. Alternatively, the apparatus may have only one table movable between exposure and measurement positions.
One of the problems with immersion lithography is the presence of bubbles in the immersion liquid. If the path of the projection beam passes through areas of immersion liquid that contain bubbles, this may deleteriously affect the quality of the patterned imaged projected onto the substrate.
Bubbles may be present in the immersion liquid for a number of reasons. The first, for example, is that not all the gas is displaced by liquid when the immersion space is filled with liquid.
Macroscopic features may prevent capillary filling of the gap between the immersion system and the substrate within the time it takes for the substrate to pass across the immersion system. This may result in gas becoming trapped in the gap. The surface tension of the liquid pulls the trapped gas volume into a bubble, which will float once the buoyancy of the bubble exceeds the surface tension of the immersion liquid holding the gas bubble to the a surface of the gap. The presence of a gap in a wall of the immersion space may provide a trap in which a bubble of gas may remain even when the space is immersed in liquid.
Rough surfaces may also prevent the capillary filling of the gap, but on a microscopic scale. The immersion liquid contacts the projections of a rough surface, but does not fully wet the contours of the surface. The extent of the roughness of the surface is proportional to the force caused by the surface tension and so gas bubbles remain trapped more easily. As the immersion liquid layer passes over the rough surface, the “effective contact angle” or the angle at which the liquid meets the surface varies more than with a smooth surface, and so gas is more likely to be trapped where the contact angle is decreased, i.e., where the distal part of projections on the surface meet the liquid before the proximal part of the projection, leaving a corner of gas at the upstream proximal part of the projection.
Secondly, for example, bubbles may form spontaneously because of a change in temperature or energy or other factors. Alternatively or additionally, gas (e.g., air) may be sucked into the system if the pressure of the system falls, e.g. with a fall in temperature. Resists and other chemicals used on the surface of substrates may cause foaming or react with the immersion liquid or radiation, causing a change in temperature or energy, or create gas bubbles chemically.
Thirdly, for example, there may be one or more gutters configured to remove excess immersion liquid from the surface of the substrate table that may also trap gas when the substrate moves relative to the immersion system or radiation system. Furthermore, these gutters may cause too much liquid to be lost, resulting in an overall drop in liquid level.
A way in which gas might not be replaced by liquid is depicted in FIG. 11a. Between a substrate W and a substrate table WT, for instance, there exists a gap that fills with liquid each time the gap passes under the immersion system 12. A gas knife 15 serves to clear the path of contaminants and liquid for the immersion system 12. However, when the liquid-filled gap passes under the gas knife 15, liquid droplets D may spray up onto the surface of the substrate W and the substrate table WT. Depending on the liquidphilic (e.g., hydrophilic) or liquidphobic (e.g., hydrophobic) nature of the substrate W surface, the surface of the droplet D forms a greater or lesser angle with the substrate surface. The liquid front F is also at an angle with the substrate W surface because the substrate W surface is traveling laterally with respect to the liquid front (the arrow indicates the direction of travel of the substrate table WT containing the substrate W). FIG. 10 shows the relative positions of the liquid front F and the droplet D. The meeting angles of the liquid may cause a small amount of gas to be trapped between the relatively moving liquid front F and droplet D surface, thus causing a gas bubble B in the immersion liquid.
Bubbles might be formed between the substrate table and the substrate, on or around sensors or on or around a closing plate used to seal the immersion system between scans of substrates. The bubbles might then detach from the surfaces and float in the immersion liquid, or even float up to the final optical element of the projection system, possibly affecting the quality of the projected image.